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Concealed beneath the streets of our great cities are thousands of miles of pipes and tunnels that carry away our wastewater. In big eastern cities such as Philadelphia, this system can date back to the nineteenth century. As urban populations have expanded and streets have been paved over, the volume of domestic sewage and contaminated rainwater after storms often overwhelms our treatment plants. These combined sewage overflows can't be allowed to back up into city streets, so they only have one place to go -- into our waterways.

Cities can spend billions maintaining and upgrading this antiquated gray infastructure -- or they can follow Philadelphia's lead and turn instead to green infrastructure. The principle is simple: instead of struggling to cope with the volume of water rushing through the sewers, prevent it from getting there in the first place by capturing it and filtering it slowly and naturally through the soil.

Over the next 25 years, Philadelphia will retrofit one-third of the paved areas serving its combined sewer system with green infrastructure that will capture the first inch of rainfall. These “green acres” are the central element in a plan that will cost $2.4 billion (a savings of billions over the all-gray alternative) and is aimed at making Philadelphia America’s greenest city.

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Lindsey Konkel is a freelance journalist based in New York City. She has a master's degree in science, health and environmental reporting from NYU, and her work has appeared at Environmental Health News, Discover magazine, Reuters, and elsewhere.

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